The present invention relates to a device for automatically locking wheels, pulleys, disks and particularly motor vehicle wheels on a horizontal-axis balancing machine.
It is known that current and more recent balancing machines for motor vehicle wheels and similar items are usually constituted by a footing which encloses the drive unit and the associated controls for the actuation of an axially hollow shaft which can rotate on bearings and is arranged so as to have a horizontal axis. A substantially dome-shaped flange is coaxially associated with one end of said hollow shaft and is centrally provided with a conical body onto which the usual central cavity of the wheel to be balanced is fitted; a cup-shaped external flange, arranged coaxially to the hollow shaft, is provided to lock the wheel to the rotating dome during rotation of the hollow shaft.
A secondary shaft is installed coaxially to said hollow shaft so that it can perform a translatory motion and act as a tie element; the end of said secondary shaft which lies proximate to the dome can be connected to (and disconnected from) said external flange, which is shaped like a cup or the like, so as to keep the wheel locked on the dome when said tie element is kept pushed towards the end of the hollow shaft which lies opposite to the wheel locking end, with a force which is sufficient to ensure the fixing of the wheel to said dome.
Several systems or devices for locking the wheel to the dome have been proposed which can make the secondary shaft automatically perform a translatory motion inside the hollow shaft towards the opposite end of said hollow shaft.
One of the best-known devices for automatically locking a wheel on a balancing machine of the above-specified type is constituted by a pneumatic cylinder which rotates together with the hollow shaft of the balancing machine; a tie element (secondary shaft arranged inside the rotating hollow shaft) is axially connected to the piston of said cylinder and is capable of applying, by means of the pressure to which it is subjected, to said tie element a force which is sufficient to lock the wheel on the locking dome or flange.
In this case, the cylinder rotates together with the shaft and it is therefore necessary to have a rotating manifold in order to convey the compressed-air supply to the cylinder; this entails an accurate and expensive construction in order to avoid disturbances and vibrations caused by said manifold.
On the other hand, advantageously there is no intrinsic limitation to the stroke of the cylinder and of the tie element; accordingly, it is possible to manufacture centering devices which are particularly practical and efficient for the user.
Another conventional device is constituted by a pack of metal springs installed on the rotating hollow shaft and arranged so as to supply the tie element with an axial force which is capable of retaining the wheel on the clamping device. When the shaft is not rotating, a nonrotating cylinder can preload the springs, so that it is possible to fit the wheel and connect to the tie element the locator bracket which will retain the wheel against the clamping dome or bracket. This device is simpler and less expensive than the previous one, but it has the drawback that it has a limited stroke of the spring. A metal spring whose dimensions are compatible with a wheel balancing machine and whose force is sufficient to clamp a wheel over a stroke of more than a few millimeters is in fact substantially impracticable.
Furthermore, the metal spring is seldom symmetrical and the various compression levels may generate imbalances which cannot be compensated.
This application, owing to the limited stroke which can be achieved on the traction element, forces scarcely practical constructive embodiments of the wheel clamping flange, with the need for adjustment or pre-clamping operations so that the limited stroke is nonetheless sufficient to retain the wheel. If one considers the clamping systems which are normally used, for the sake of the best practicality it is in fact desirable to have a traction element stroke on the order of 70-100 mm, with a minimum force on the order of 250 kg. These results cannot be achieved with conventional systems that use the reaction of springs.